The copyright and trademark laws protects IPs and Dead by Daylight is an IP. By law selling fan goods of any IP is technically illegal and the copyright holder of the IP is capable of going after those who do.
Basically, this is how it works: If you create any fanart for profit, you need the permission of the copyright holder. That goes for anything that is copyrighted and trademarked.
Dead by Daylight has always been supportive of Fan Art of any kind (e.g. they always retweet Fan Art or show Fan Art on streams), but selling it is something different than showing your appreciation with creating Fan Art and putting it online.
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